Nicotinamide adenine dinucleotide (NAD+) was discovered about 120 years ago as a coenzyme that mediates ethanol fermentation. It is a classical coenzyme present in all species and plays a central role especially in redox reactions.

NAD+ is not permeable to cell membranes and has no transporter in mammals, including humans, so it cannot be directly absorbed or taken up as a nutrient.

Therefore, NAD+ is biosynthesis from dietary tryptophan, nicotinamide (NAM) and nicotinic acid (NA), as well as nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR), is performed in vivo.

NAM and NA are collectively known as niacin (water-soluble vitamin B3).